Hot melt adhesives are solid at room temperature but upon application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form. The hard phase(s) formed upon cooling the adhesive imparts all of the cohesion (strength, toughness, creep and heat resistance) to the final adhesive. Curable hot melt adhesives, which are also applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction. An advantage of hot melt curable adhesives over traditional liquid curing adhesives is their ability to provide “green strength” upon cooling prior to cure. Advantages of hot melt curable adhesives over non-curing hot melt adhesives include improved temperature and chemical resistance.
The majority of reactive hot melts are moisture-curing urethane adhesives. These adhesives consist primarily of isocyanate terminated polyurethane prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane/urea polymer. Polyurethane prepolymers are conventionally obtained by reacting polyols with isocyanates. Additives are commonly included in reactive hot melt adhesive formulations to provide improved properties, such as improved green strength before solidification and increased cure speed. Cure is obtained through the diffusion of moisture from the atmosphere or the substrate into the adhesive and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is a crosslinked material polymerized primarily through urea groups and urethane groups.
Moisture curing adhesives are generally prepared in a one-step polymerization method. While two step-polymerization method are know in the art, such as the processes disclosed in U.S. Pat. No. 6,365,700 and U.S. Pat. No. 7,074,297, there continues to be a need in the art for novel polymerization manufacturing processes for forming moisture reactive hot melt adhesives. The present invention addresses this need.